It takes a lot for a TV show to truly surprise you. Not in the one-off moments like the Red Wedding or episodes like The Fly, but on a much broader level of both premise and execution.
Going into shows such as Friends, Mad Men, or The West Wing, you don't know exactly what's going to happen on an episode-by-episode basis or where certain characters might end up, but there's an expectation as to what the show will be like in terms of storytelling, plotlines, and tone that they typically meet.
It's especially the case now, when the landscape is full of reboots and revivals and series are hyped to death before airing, but there are occasions when you thought you knew what you were getting into, only for it to completely subvert all expectations.
While anyone referring to Buffy the Vampire Slayer now means the TV show unless otherwise stated, it was a different case back in the 90s. Joss Whedon's original attempt at getting the project off the ground didn't go as planned, resulting in a 1992 movie that had little in common with his script and played as a broad comedy.
Five years later, then, when a TV show of the same name debuted, the movie version was still in the cultural consciousness. Anyone expecting similar high school silliness would've been caught off guard, with the series darker, smarter, more action-packed, and on a completely different level quality wise.
However, even The WB's attempts at marketing the series didn't quite nail it: the trailer for the series premiere really overplayed the horror elements, making this look like a cheap slasher movie where everyone would die. In neither the movie nor the trailer was there any of the brilliant Whedon dialogue, pop-culture references, or sense of character that would make the show so beloved to each generation that watches it.